Changes in intellectual functioning in the aged is a serious public health concern that has far reaching social and economic consequences. Recent research has demonstrated that cognitive changes observed in aging are caused by mental slowing. Therefore, understanding the causes of cognitive slowing in the aged is necessary for understanding changes in intellectual ability that occur with advanced age. Important long-term objectives are to elucidate the nature and mechanisms of age-related cognitive slowing; improve our ability to identify brain correlates of cognitive slowing in the aged; provide needed information for identifying risk factors predictive of pathologic cognitive slowing; and distinguish between normal age-related cognitive slowing and that caused by dementing disorders, such as Alzheimer's Disease. Specific aims are to (1) determine whether certain kinds of mental operation are more susceptible to the effects of aging than others; (2) determine whether certain types of cognitive processing are exempt from the effects of aging; (3) determine whether all aged individuals show equivalent amounts of cognitive slowing; (4) determine to what extent cognitive slowing in the aged is due to increased processing variability, by formal (mathematical) modeling of the response characteristics of individuals; (5) test the major competing models of cognitive aging, the Generalized Slowing (task-independent) and interactive (task-specific) models; and (6) provide a conceptual and analytic framework that can be used by future studies to clarify whether significant age differences reflect change in a specific cognitive function or are simply due to a global decline in information processing speed. Experimental methods that permit the measurement of the rate of executing basic mental operations will be used in a sequence of four inter-related studies. Competing models of cognitive aging and theory-based predictions will be tested using regression techniques. Data from individual subjects will be modeled and analyzed using the ex-Gaussian distribution function to distinguish the effect of aging on mental slowing from its effect on performance (intra-subject) variability in aged individuals. The findings from this investigation will advance experimental research techniques so that they can be used in clinical settings to characterize and identify changes in cognition that are indicative of age-associated pathologic conditions, such as Alzheimer's Disease. The results will provide information needed for pharmacologic testing and treatment, and to correlate with physiologic, biochemical and pathologic changes in aging.